Re-directing post for former inhabitants?

[Ducks]

We got something today for the former owner of our property. It looks official and important, so we'd like to get it to them. Can we just cross out the address, write their new address on it, and drop it into the post office? 

What is procedure here for incorrectly/out-of-date addressed mail?

[TamaraEnLaPlaya]

Some of the official post has instructions on the back for undelivered mail where you tick boxes to indicate 'moved', 'not known', etc and space to write a new address. I've filled these in before and just handed them back to the postman. Whether they go on to the new address (when appropriate) or back to the sender I don't know.
You could always pop it inside another envelope and just post like normal if you think it's important - I'd be very happy if someone did that for me! (Unless it was the taxman trying to chase me down ...  ) Pay it forward!